The present invention relates to systems for controlling the stability of a vehicle, commonly known as ESP (Electronic Stability Program) systems.
In safety systems for vehicles it is necessary to be able to assess the behavior of the vehicle in real time. This is the basis of the so-called ESP systems for controlling stability. These systems currently rely on, inter alia, monitoring movements of the vehicle by installing sensors to measure the transverse acceleration of the vehicle and the yaw rate of the vehicle.
When moving under good safety conditions, that is, when the stability of the vehicle is not compromised, the vehicle obeys the driver's commands. If the driver, basically as a result of his handling of the steering wheel, drives the vehicle beyond the limits of stability, the vehicle will exhibit oversteering or understeering. The vehicle turns, that is, performs a yaw movement, in excess of that desired by the driver (oversteering) or less than desired by the driver (understeering).
Using a mathematical model of the tire and a mathematical model of the vehicle, and based on measurements supplied by sensors recording the actions of the driver of the vehicle (angle relative to the steering wheel, application of the brakes and accelerator) and speed sensors for the wheels, and from measurements of the transverse acceleration and yaw rate, an ESP system constantly calculates the forces at the center of the wheels and estimates the grip potential of the road surface as a function of the transverse acceleration. Furthermore, the ESP system evaluates the behavior of the vehicle, compares it to the behavior desired by the driver, and corrects this behavior if it establishes that the vehicle is not moving along a stable path.
However, the use of tire models can introduce a certain number of approximations into the overall model. Furthermore, the fact that a control system is based on the displacements of the vehicle necessarily leads to a response a posteriori, which can be effective only after a delay depending on the inertia of the vehicle. It can be seen from this that an ESP system, since its variables of state include measurements of the transverse acceleration and the yaw rate of the vehicle, first of all has to measure the displacement of the vehicle before deciding whether the displacement is within the bounds of stability or not, and can only then act on the operating means of the vehicle. Moreover, the currently available ESP system observes only the movements of the body of the vehicle without knowing specifically the exact reasons for the loss of control. The movements of the body of the vehicle are caused by contact between the tire and the ground.
The system will detect a displacement of the vehicle not in accord with the command given by the driver, more slowly the greater the inertia of the vehicle, and the necessary correction will be all the more difficult the greater the inertia. At the present time the operating means are basically the vehicle's brakes, controlled in this case wheel by wheel and outside the voluntary action of the driver, and the motive force, which can be reduced automatically by regulating the engine.
Furthermore, the detection of yaw movements requires the use of costly sensors. Also, existing systems have to estimate the grip of the wheels on the road surface in order to select the actuating parameters. This estimation deviates to a greater or lesser degree from the actual conditions.
The object of the present invention is to obviate the aforementioned disadvantages and, more particularly, to exclude completely the inertia of a vehicle in order to be able to act on the appropriate operating means so as to maintain the vehicle in a stable path in accordance with the driver's commands, by regulating the operating means in such a way that the actual forces acting at the center of each wheel correspond to the desired forces.
The invention provides a vehicle stability control system and a method for controlling the stability of a vehicle that have the advantage that they can be carried out without having to measure the yaw angle of the vehicle.
The invention relates to a vehicle comprising a body and at least one front ground contacting arrangement and at least one rear ground contacting arrangement, each ground contacting arrangement comprising in each case one wheel, each wheel comprising a pneumatic or non-pneumatic tire in contact with the ground, the vehicle having a characteristic time that is a function of its inertia and corresponds to the time phase shift in the manifestation of the cornering forces on the wheels in the front and in the rear, following a command from the driver of the vehicle, the vehicle being provided with operating means to act on the forces transmitted to the ground by each of the wheels.
In a vehicle the steering of the wheels produces a cornering force at the front, a movement of the vehicle body, followed by a cornering force at the rear. The cornering force of the rear wheel or wheels thus intervenes with a slight delay with respect to the command on the steering wheel. In order to establish more precisely the actions required to correct the path, the invention proposes to take into account this delay T as explained hereinafter.
According to a first aspect of the invention, the method comprises the following steps:                (a) measuring in real time the actual value of one variable selected in the group of the cornering force “Y” and the vertical load “Z” acting at the center of each of the front and rear wheels;        (b) calculating in real time the desired value of at least one reference parameter, said at least one reference parameter being correlatable to the actual value, as a result of an action of the driver on the operating means and taking into account the load transfers on both sides of the mid plane of symmetry of the vehicle;        (c) comparing said desired value of the reference parameter of step (b) to the actual value to determine whether the actual value is compatible with the desired value of the reference parameter; and        (d) if the comparison of step (c) indicates that the actual value is not compatible, acting on the operating means such that the actual value is brought into substantial compatibility with the desired value of the reference parameter.        
A preferred aspect relating to the specific application of the invention to vehicles each of whose axles comprises at least two ground contacting arrangements each comprising one wheel, is described hereinafter, the ground contacting arrangement being mounted on either side of the mid-plane of symmetry of the vehicle. This is the conventional arrangement in a four-wheeled touring vehicle. However, the invention is also applicable to two-wheeled vehicles, such as motorbikes, being noted that in this case the inertia of the body is considerably lower. Each ground contacting arrangement comprises a wheel, generally having a tire, which in this description means a pneumatic tire or non-pneumatic tire, in contact with the ground. The vehicle is provided with operating means to act on the forces transmitted to the ground by each of the wheels, such as brakes, means for steering the wheels, optionally selectively wheel by wheel, and distribution of the loads carried by each of the wheels.
The commands of the driver of the vehicle are intended to maintain the vehicle on a straight line path regardless of the ambient disturbances (for example sidewind gusts, change of the road grip on all or part of the vehicle), or are intended to cause the vehicle to execute a transverse displacement (change of lane for overtaking on a motorway) or to turn. Regardless of the operating means of the vehicle that are actuated by the driver (conventional steering wheel, operating lever as illustrated for example in patent application EP 0 832 807), the driver's wish in fact is to exert specific cornering forces or specific changes of these cornering forces.
The invention thus proposes to measure in real time the effective cornering forces, compare them to commands of the driver translated into cornering forces or changes in cornering forces, and thereby to control appropriate operating means available on the vehicle. In a first particular embodiment, said variable is the cornering force “Y” and said desired value of at least one reference parameter of step (b) is the desired cornering force “Yd” at the center of each wheel. More particularly, step (c) further comprises generating an error signal representative of the magnitude and direction of the difference between the actual and desired cornering forces and step (d) comprises controlling said operating means to minimize said error signal.
In another particular embodiment, said variable is the cornering force “Y”, said operating means including a command for controlling the steering, step (a) comprises calculating in real time the effective yaw moment corresponding to the actual cornering forces “Y”, said desired value of at least one reference parameter of step (b) being the desired yaw moment, step (a) comprises measuring in real time a signal at the steering command and calculating the desired yaw moment “Md”, and step (c) comprises utilizing said desired yaw moment “Md” for comparison with the effective yaw moment of step (a). More particularly, step (c) further comprises generating an error signal representative of the magnitude and the direction of the difference between the effective yaw moment and the desired yaw moment “Md”; and step (d) comprises controlling said operating means to minimize said error signal.
Accordingly, if the cornering force of the front axle has been saturated, the vehicle will understeer since the cornering forces of the front train are less than the forces desired by the driver (desired forces meaning forces corresponding to the actions by the driver on his steering wheel or on other steering commands available). An automatic action, for example of the type already known per se in conventional ESP systems (other types of actions will be discussed hereinafter) enables a resultant force to be exerted on the vehicle chassis in accordance with the driver's wishes and thus enables understeering to be avoided.
If on the other hand it is the cornering force of the rear axle that first becomes saturated, then the vehicle will oversteer since the cornering forces of the rear train are less than the forces desired by the driver. The automatic action enables a resultant force to be exerted on the vehicle chassis in accordance with the driver's wishes and thus enables oversteering to be avoided.
The above description refers to what is conventionally called the stationary state (or steady state). When considering a typical transient state involved in an emergency maneuver (avoiding an obstacle, changing lane), the speed of engagement of the steering wheel may be regarded as equivalent to a desired yaw moment acting on the vehicle. If the actual yaw moment is less than the desired yaw moment, the vehicle will not turn sufficiently. If on the other hand the actual yaw moment is greater than the desired yaw moment, the vehicle will turn too much.
According to yet another particular embodiment, said variable is the vertical load “Z”. More particularly, said operating means including a command for controlling the steering, and said desired value of at least one reference parameter of step (b) being the desired load “Zd” at the center of each of the front and rear wheels, the method comprises a step for measuring in real time a signal at the steering command and calculating the desired loads “Zd”. More particularly, step (c) further comprises generating an error signal representative of the magnitude and the direction of the difference between the actual loads “Z” and the desired loads “Zd”; and step (d) comprises controlling said operating means to minimize said error signal.
The method according to the invention permits, if the cornering forces of one of the axles do not correspond to the desired cornering forces, or if the effective yaw moment is greater than the desired yaw moment, or if the vertical loads do not correspond to the desired vertical loads, the transmission of an action signal to the operating means in order to minimize the error signal without the need to establish such a signal, without the need to measure the yaw rate of the vehicle. Of course, such a method is compatible with measuring the yaw rate, particularly if it is desired to add redundancy terms to the calculations.
As can be seen, the invention provides a method for regulating a system for controlling the stability of a vehicle based on the forces acting at the center of each wheel of the vehicle. More specifically, the actions of the driver, whether they involve steering, accelerating or braking, will be reflected in forces (changes in forces) transmitted by the tires to the ground. Depending on whether or not these force variations are compatible with respect to the commands of the driver, it may be concluded whether or not the vehicle is stable. The origin of future displacements is found starting from the forces acting on the ground. In this way it is possible to correct the path of the vehicle much sooner and an ESP system, or more generally a stability control system, gains in fineness of correction. Both the safety and comfort of the driver and passengers are improved.
The estimation of stability criteria in real time, based on forces acting on the ground, enables the stability control of the path of a vehicle to be improved, the direct measurement of the force enabling, for example, the saturation point of the tire on each of the wheels to be monitored accurately regardless of the grip on the road surface, by detecting the occurrence of non-linearity between the developed cornering force and the sideslip angle of the tire in question, as well as non-linearity of the developed cornering force and the load applied to the tire.
The cause of loss of stability of the vehicle is mainly the fact that the tires are no longer able to correct the path, given the movement of the vehicle. Irrespective of the cornering force developed by the tires, this will never be able to counteract the forces of inertia. This may be due to a poor grip (wet road, (black) ice, snow, sand, dead leaves), to the fact that the tire is used by the driver under improper conditions (flat tire or underinflated tire), or to the fact that the vehicle is directly placed in a situation of excessive drift or sideslip that exceeds the physical limits of one or more of the tires. In this case it may be said that one or more of the tires reaches its saturation point.
The suspension bearings may be equipped with instruments, as proposed in patent application JP60/205037, which enables the longitudinal and transverse forces developed by the tire to be recognized easily by measurements made on the suspension bearings. Alternatively, the tire itself is equipped with sensors for recording the forces of the tire on the ground. A measurement may for example be made as explained in patent DE 39 37 966 or as discussed in U.S. Pat. No. 5,864,056 or in U.S. Pat. No. 5,502,433.
On the basis of the forces measured by one or other of the above methods, and from equilibrium equations of a ground contacting arrangement, the forces acting at the center of each wheel may accordingly easily be calculated. Thus, in real time 3 forces X, Y and Z are available, which in particular enables the Y or Z signal to be processed for the reasons explained in the present document.
The invention also relates to vehicle stability control systems, said vehicle having a body and at least one front ground contacting arrangement and at least one rear ground contacting arrangement, each ground contacting arrangement comprising in each case one wheel, each wheel comprising a pneumatic or non-pneumatic tire in contact with the ground, the vehicle having a characteristic time that is a function of its inertia and corresponds to the time phase shift in the manifestation of the cornering forces on the wheels in the front and in the rear, following a command from the driver of the vehicle, the vehicle being provided with operating means to act on the forces transmitted to the ground by each of the wheels, such as brakes, means for steering the wheels. The system further comprises:                (a) means for measuring in real time the actual values of one variable selected in the group of the cornering force “Y” and the vertical load “Z” acting at the center of each of the front and rear wheels;        (b) a controller allowing to calculate in real time the desired values of at least one reference parameter, said at least one reference parameter being correlatable to the actual values, as a result of an action of the driver on the operating means and taking into account the load transfers on both sides of the mid plane of symmetry of the vehicle, said controller allowing to perform comparisons between the desired values with the measured actual values in order to obtain an error signal, and;        (c) means for acting on the operating means so as to minimize the error signal.        
According to various aspects, as explained hereabove for the method for controlling the stability of a vehicle, the variable can be the actual cornering force “Y” in which case the reference parameter can be either the desired cornering force “Yd” or the desired yaw moment “Md”, or said variable is the vertical load “Z” and the reference parameter is the desired loads “Zd”.